Filamentous hemagglutinin (FHA) is a 220 kD, non-fimbrial surface associated protein produced and secreted by Bordetella pertussis (BP). It is a necessary factor for BP to adhere to ciliated respiratory epithelial cells during whooping cough, Tuomanen et al., J. Infec. Dis. 152:118-125 (1985). FHA has also been shown to interact with the integrin complement receptor 3 (CR3) on macrophages and other leukocytes, Relman et al., Cell 61:1375-1382 (1990). CR3 is also known as Mac-l,.alpha..sub.M .beta..sub.2 and CD 11b/CD18. Distinct portions of FHA are responsible for its binding to ciliated respiratory epithelial cells and to leukocytes.
BP binds to two cell types during infection: leukocytes and ciliated cells. Adherence to cilia of the ciliated cells depends on recognition by BP of carbohydrates such as galactose containing glycolipids, Tuomanen et al., J. Exp. Med. 168:267-277 (1988).
The BP organism binds to leukocytes by two means. For the first, it binds to the integrin CR3, a step which precedes entry of the bacteria into the leukocyte, as discussed in more detail below. For the second, BP binds to leukocyte carbohydrates. This carbohydrate binding is analogous to that when BP adhere to cilia. Galactose is the minimum requirement for a carbohydrate receptor. It is found, for example, in such blood group determinants as Lewis a.
There are two aspects of a BP infection. One is the invasion of the leukocytes which takes place when BP binds to the integrin of leukocytes. It is a protein/protein interaction. The other aspect is adhesion of BP to the leukocyte or the cilia through a protein/carbohydrate interaction.
The FHA gene of BP has been sequenced, Relman et al., Proc. Natl. Acad. Sci. U.S.A. 86:2637-2641 (1989) and Domenighini et al., Molec. Micro. 4:487-800 (1990), and a number of expression products have been produced, Delisse-Gathoye et al., Infect. Immun. 58:2895-2905 (1990).
FHA and the integrin on leukocytes interact in a protein-protein recognition event. The interaction between FHA and leukocyte integrin involves recognition of the arginyl-glycyl-aspartyl sequence at amino acid positions 1097 to 1099 in FHA. This sequence will hereinafter be identified as the RGD sequence or simply, RGD, and the region of FHA or FHA segment on which it occurs as the RGD region. R, G and D are the standard one letter abbreviations for arginine, glycine and aspartic acid.
It has long been known that leukocytes can invade or pass through vascular endothelial tissue by a process in which integrins, such as CR3, bind to receptors for integrins on the surface of endothelial cells as a step in a sequence of reactions which results in a widening of the junctions between such cells to permit passage by the leukocytes. The receptors for CR3 on endothelia are referred to herein as integrin receptors. There may be one or more than one such integrin receptor.
Another molecule which binds to the integrin CR3 is the serum complement component C3bi, an opsonin. Bacteria which have the C3bi component fixed to their surface by any means, such as a binding event during a host recognition process, are bound to the leukocyte integrin in such a way that the bacteria are taken up and killed by the leukocyte.
In addition to the ability to recognize C3bi, FHA and integrin receptors on endothelial cells, CR3 also binds to Factor Ten of the coagulation cascade (Altieri et al., Science 254:1200-1202, 1992). The coagulation cascade is involved in inflammation since procoagulant activity arises on endothelial cells during infection or other noxious stimuli. Three regions of Factor Ten participate in recognition of CR3.
To summarize, FHA, C3bi, Factor X and endothelial cell integrin receptors are molecules with binding regions for CR3.
A particular infection in which leukocyte mediated damage contributes to morbidity and mortality of disease is bacterial meningitis. Depending upon the infecting organism, thirty percent of the cases of meningitis per year die despite sterilization of the infection by antibiotics. Over fifty percent of the survivors have permanent and severe sequelae such as paralysis, deafness, and learning disabilities. Obviously, the prevention and/or diminishment of such damage would greatly enhance the quality of life for the survivors of this disease.
Activated leukocytes also contribute to cerebral edema and blood-brain barrier injury. Neutropenic animals (animals in which the leukocytes have been artificially diminished) have been found to have improved survival rates in experimentally induced disease. A high amount of inflammation in the subarachnoid space correlates directly with a poor outcome of disease. Inhibition of the accumulation of leukocytes in cerebrospinal fluid directly correlates with improved morbidity and mortality of experimental pneumococcal meningitis and of Haemophilus influenzae meningitis and bacteremia in children.
Clearly, an agent which would inhibit the influx of leukocytes into infected sites would be a therapeutic tool of immense value particularly if non-leukocyte mediated defense systems are left functionally intact. It would further be beneficial to block leukocyte diapedesis only at inflamed sites and not at other sites throughout the body. Thus, treatment directed at inflamed endothelia would be advantageous over that directed to leukocytes.
The use of antibiotics magnifies the deleterious effects of inflammation during infectious diseases. This is due to the mechanism by which such agents exert their antiinfective effects. For example, following the administration of a beta-lactam antibiotic (or another cell-wall directed antibiotic), the bacteria disintegrate due to lysis by the antiinfective agents. The resulting fragments of bacteria initiate a dramatically enhanced inflammatory response. Earlier research has indicated that inhibition of this enhanced level of inflammation correlates with improved morbidity and mortality, Tuomanen et al., J. Infect. Dis. 155:985-990 (1985) and Kadurugamuwa, Program and Abstracts of the 27th ICAA Meeting, p. 205 (1987). In penumococcal meningitis, for instance, mortality can be directly correlated with the amount of meningeal inflammation, McAllister et al., J. Infect. Dis. 132: 355-360 (1975). Thus, a method of dampening inflammation during the course of therapy with an antibiotic would be advantageous in treating infections, particularly meningitis, septic arthritis, and endophathalmitis.